The present invention relates generally to charged particle pulse generation, and more particularly to the generation of intense beam pulses of short duration via the coupling of large amounts of magnetic energy from long-rise time current sources to high voltage intense charged particle beams by magnetoplasmadynamic action.
Intense particle beams are used in nuclear weapon effects simulation, inertial and magnetic confinement fusion research, laser pumping, microwave production and possible advanced weapons systems. The conventional technique for producing intense electron and/or ion beams is to use a capacitive pulse forming line to provide a short duration, high voltage pulse to a pair of electrodes forming a diode. Systems of this type have been built and operated at megajoule energy levels, voltages of several megavolts and currents in excess of a megampere. However, such systems are quite large, and quite expensive due primarily to the electric field strength limitations on capacitive energy storage.
Inductive energy storage is an alternative to the capacitive energy storage typically utilized to generate voltage pulses. Such inductive energy storage systems are limited only by the mechanical strength of the conductors in the system and can exceed the energy storage density of capacitive systems by factors of a few thousand. Typically, in such systems a primary energy source such as rotating electrical machinery (homopolar generators, or pulse alternators, for example) or magnetodynamic systems (magnetic flux compression generators, or pulsed MHD devices) may be utilized to supply current to a storage inductance. The use of such rotating electrical machinery as the primary energy source is especially advantageous in that such machines require significantly less volume than capacitor banks and are thus extremely compact. However, such current sources typically have rise-times on the order of 10.sup.-1 -10.sup.-4 seconds which are much longer than the operating times of intense beam diodes (10.sup.-6 -10.sup.-7 seconds). Since these rise-times are much longer than that desired for driving intense beam diodes, the current must be carried by a separate auxiliary element during the time required for delivery of energy to the inductive store. Current flow in this auxiliary element must then be interrupted in order to direct energy into a diode connected in parallel with the auxiliary element for the generation of the actual intense beam pulse. For further discussion on this point, see the reference Pulsed High Magnetic Fields, by Heinz Knoepfel, American Elsevier Publishing Company, 1970 Chapter 6.
The use of auxiliary elements (opening switches) to match slow rise-time current sources to electrical loads requiring fast rise-times is well-established over a wide variety of time scales (10.sup.0 -10.sup.-7 seconds). Such auxiliary element opening switches have typically found application in capacitive systems as a means of pulse-sharpening. In particular, for intense beam systems, a switch element has been developed called a plasma-erosion switch, in which energy from a high voltage capacitive-pulseline is prevented from reaching the beam diode until electric fields in the switch gap deplete (or erode) ions from a puff of plasma injected by a small pulse plasma source. The purpose of such a switch is to prevent a capacitively-coupled prepulse of high voltage from disturbing the initation of current flow in the beam diode. This plasma erosion switch has also been utilized to shorten the effective pulse rise-time of high power capacitively-driven pulsers.
A different type of auxiliarly opening switch is based on the principle of expoding wires or foils. These expoding wires are designed such that the thickness of the conductor will be smaller than the skin depth of the current to flow therethrough. If the vaporisation of the wire metal is fast enough, there will be a certain time interval during which the electrical conductivity will be very low, i.e., the switch will be open. However, the problem with utilizing such an expoding wire switch, is that it frequently requires a significant fraction of the stored energy in order to vaporize the wire switch. Additionally, these switches are not reuseable.
A still further design for an auxiliary opening switch utilizes an SCR switch configuration. This SCR switch configuration has been proposed by Los Alamos National Laboratory and is composed of a matrix of 36.times.36 SCR switches connected in various series and parallel combinations to give the proper voltage and current characteristics for switching. It is estimated that this switch will cost on the order of one-half million dollars.
At this point, the limitations and disadvantages of the prior art systems will be summarized. Beginning with capacitive-driven pulselines, systems based on such pulselines require large volume energy storage with attendant physical support requirements and increased cost. Additionally, such capacitive pulseline systems require synchronized switching of high precision in order to deliver high power flows to the beam diode via multiple switch gaps. Finally, such pulseline systems often depend on special low impedance load characteristics in order to achieve vaccum magnetic insulation for high power flux to the beam diode.
Inductive storage and switching systems involve either destructive switching elements or complex exposive breakers and/or solid state elements to match electromechanical current sources to beam diode operating requirements. Additionally, in order to achieve short switch opening times, only short conduction times on the order of microseconds are available. High voltages are thus required to deliver energy to the inductive store in short times. These current conduction times are too short to permit utilization of significant inductive energy storage without high power capacitively-driven pulse lines. The use of such capacitive pulselines would erase the advantage of an inductive energy storage stage.